In the past, it has only been with difficulty that the shallow water range of the depth sounder can be brought below, for instance, 2.6 feet. As boaters are used to navigating in water having a depth of only inches, this limitation on the shallow water response of the depth sounder is troublesome.
In essence, for all depth sounding systems in which acoustic pulses are projected into the water by a transducer, the depth is limited by the length of the transmitted pulse, and the response or settling time of the receiver elements and the transducer itself. For instance, for a 200 micro-second pulse, the equivalent depth is about 0.5 feet. If one is not to detect the trailing edge of the transmitted pulse, one cannot open up the receiver section of the depth sounder earlier than 200 micro-seconds; or equivalently half a foot. Additionally, transducer ring, after activation, is approximately 80 micro-seconds and at 200 KHZ, and transducer activation takes 50 micro-seconds. More importantly, receiver tank circuits and circuit delays account for almost the equivalent of two additional feet in delay. The result is a loss of almost 2.6 feet off the range of most depth sounders. This, therefore, describes a fundamental shallow water limitation on all depth sounding systems.
In an effort to improve the shallow water performance, depth sounders have been given shorter and shorter transmit pulses, as well as reduced output power. In so doing, while it is possible to obtain a depth sounder which will provide readings down to 0.9 feet, the shortening of the transmit pulse and the power reduction makes the entire system extremely unstable and subject to not only ignition noise, but also to turbulence, weeds, under-water growth and sharp underwater structure changes.
In summary, the shallow water response of the depth sounder is limited to the transmit pulse length at the very least, and more importantly to the ringing time or stabilization time of the circuits utilized to receive the sonic pulses.